The present invention relates to a power circuit for use in a device which can be operated by a secondary battery, such as. a portable device of a notebook type personal computer or the like.
A device which can be operated by a secondary battery, such as a portable device of a notebook type personal computer or the like, is normally provided with a power circuit which receives a commercial electric power via an AC adapter and supplies electric power for charging the secondary battery and driving the device.
FIG. 9 is a block diagram schematically showing the construction of a notebook type personal computer (referred to as a "notebook computer" hereinafter) which is an example of the prior art devices having such a charging function. When externally supplying the commercial electric power to this notebook computer 100, the notebook computer 100 is connected to an AC power line of a commercial power of 100 V via an AC adapter 90. The notebook computer 100 is constructed of a power circuit section having a charging circuit 104, a battery pack 106 and a selecting circuit 108 and a system section 110.
In the above-mentioned construction, a DC voltage outputted from the AC adapter 90 is supplied to the charging circuit 104 and the selecting circuit 108. The charging circuit 104 is a circuit for supplying a charging current to the battery pack 106, and charging is executed with a voltage of 8.4 V (note that the maximum current is not greater than 1.5 A) taking the characteristics of the secondary battery used in the battery pack 106 and so on into consideration. The selecting circuit 108 is connected to the AC adapter 90 and the battery pack 106 and operates to select between the AC adapter 90 and the battery pack 106 for supplying a power to the system section 110. This selecting circuit 108 is constructed of so-called the "matched diodes" D1 and D2 as shown in FIG. 10, and a voltage supplied from the AC adapter 90 is set so as to be greater than the voltage supplied from the battery pack. Therefore, when the AC adapter 90 is connected to the notebook computer 100 in order to supply commercial electric power to it, the commercial electric power is supplied to the system section 110. When the AC adapter 90 is not connected, electric power is supplied from the battery pack 106 to the system section 110. The system section 110 has a main board 114 for putting the original functions of the notebook computer into effect as well as a DC-to-DC (DC/DC) converter 112 for generating a power voltage necessary for the main board 114 from the voltage supplied from the selecting circuit 108.
For the purpose of concurrently supplying an electric power from the AC adapter 90 to the charging circuit 104 and the system section 110 in the above-mentioned construction, it is necessary to make the capacity of the AC adapter 90 greater than the sum of the maximum values of the powers required by the sections of the notebook computer. In the above-mentioned prior art case, since the maximum electric power required by the charging.circuit 104 is 8.4 V.times.1.5 A=12.6 W and the maximum electric power required by the system section 110 is 12 W, there is required a capacity of 12.6 W+12 W=24.6 W, consequently needing a large-size AC adapter. On the other hand, a compact portable device having a charging function has a limited size and capacity of a cooling mechanism provided inside its casing, and therefore, the calorific value is required to be suppressed. However, when executing charging during the operation of the system by concurrently supplying a power to the charging circuit 104 and the system section 110, the calorific value increases, and this requires an increase in size of the cooling mechanism. Therefore, in a device of the current notebook computer or the like having a charging function, there is a normal practice of executing no charging in the operating state of the system, and the operation of the system section 110 is stopped when charging the secondary battery in the battery pack 106.